Equalizing transmitter and method of operation

ABSTRACT

A transmitter for providing channel equalization that includes a first driver and second driver having a high pass filter. The first driver generates a first output signal representing a digital input signal. The second driver generates a second output signal representing a high pass filtered version of the digital input signal. The first and second output signals are summed to provide a third output signal that is channel equalized for transmission over a channel.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/827,619, filed Aug. 17, 2015, which is a continuation of U.S.application Ser. No. 12/522,308, filed Jul. 7, 2009, which is the U.S.National Stage Application of International Application No.PCT/US2008/000195 filed on Jan. 7, 2008, which claims the benefit ofU.S. Provisional Application No. 60/879,443, filed Jan. 9, 2007. Theentire teachings of the above applications are incorporated herein byreference.

BACKGROUND

In a conventional digital transmission system, a transmitter sendsdigital signals by setting a signal parameter of an output signal suchas the current or voltage of the output signal to one of a plurality ofdiscrete values during each of a succession of intervals referred toherein as data intervals. The value of the parameter during each datainterval denotes a digital value being transmitted during that datainterval. For example, in a binary system, the transmitter may set thesignal parameter to a first value when a digital 1 is being sent and toa second, different value when a digital 0 is being sent. The outputsignal from the transmitter passes to the receiver over a channel orsignal path. The signal typically experiences distortion as itpropagates through the signal path from the transmitter to the receiver.One type of distortion arises from temporal spreading of the signal,which results in the signal parameter during a given data interval, asreceived by the receiver, being affected by the signal parameter duringother data intervals. This effect is referred to as inter-symbolinterference (ISI). As further discussed below, ISI makes it moredifficult, or impossible, for the receiver to determine the value of thesignal parameter during each individual data interval. The distortionwhich causes ISI may arise from frequency dependent attenuation in thesignal path. Typically, signal components at higher frequencies areattenuated to a greater degree than signal components at lowerfrequencies. It is desirable for a transmission system to compensate forthis frequency dependent attenuation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E illustrate the effects of frequency dependent attenuation.

FIG. 2 is a block diagram of a transmitter equalizer in accordance withan embodiment of the present application.

FIG. 3 and FIG. 3A are a circuit diagram of the transmitter equalizer ofFIG. 2.

FIG. 4 is a diagram showing the operation of a component in thetransmitter equalizer of FIGS. 2 and 3.

FIG. 5 is a diagram showing the operation of a component in thetransmitter equalizer of FIGS. 2 and 3.

FIG. 6 shows graphs of certain signals in the operation of thetransmitter of FIGS. 2 and 3.

FIG. 7A is a graph of the frequency response illustrating a channelattenuation.

FIG. 7B is a graph showing the frequency response of the transmitterequalizer shown in FIGS. 2 and 3.

FIG. 7C is a graph showing the combination of equalization and channelattenuation.

FIG. 7D is a graph showing an eye diagram for a pseudo-random bitsequence transmitted without equalization.

FIG. 7E is a graph showing an eye diagram for the same pseudo-random bitsequence shown in FIG. 7D, but transmitted using the equalizer of FIGS.2 and 3.

FIGS. 8A through 8E show the frequency response of a channel, filter,and equalized channel, and eye diagrams with and without equalization,but for a different channel response and different equalizationadjustment than shown in FIGS. 7A through 7D.

FIGS. 9A through 9E show the frequency response of a channel, filter,and equalized channel, and eye diagrams with and without equalizationfor yet another channel response and different equalization adjustment.

FIG. 10 is a block diagram of a system incorporating the transmitter ofFIG. 2 in accordance with an embodiment of the present application.

FIG. 11 is a block diagram of an adjustable transmitter in accordancewith another embodiment of the present application.

FIG. 12 is a block diagram of a transmitter in accordance with yetanother embodiment of the present application.

DETAILED DESCRIPTION

Certain effects of frequency dependent attenuation are diagrammaticallyillustrated by FIGS. 1A-1E. FIG. 1A depicts a series of digital values11101101 in a binary system. FIG. 1B depicts in solid lines a signalsent by a typical transmitter representing the series of digital valuesshown in FIG. 1A. For the first three data intervals T_(B) (alsoreferred to as “bit intervals”) the signal parameter as, for example,current or voltage, remains at steady state at a first discrete value V₁representing the digital value 1. At the beginning of the fourthinterval, the signal parameter changes abruptly, over a transition timeT_(t) much less than a bit interval T_(B), to the second discrete valueV₂ representing the digital value 0. At the beginning of the next bitinterval, the signal parameter returns abruptly to the first value V₁representing digital 1, and so on. Stated another way, at eachtransition between bit intervals having different data values, thesignal parameter approximates a perfect square wave having a “swing” ordifference between values S=(V₁−V₂), the wave being centered on atransmitter threshold value H_(T) midway between the first and seconddiscrete values V₁ and V₂. Although the signal may or may not beperiodic depending on the content of the data being transmitted, thesignal can be regarded as having a data rate of 1/T_(B) and a bit togglefrequency f=1/(2T_(B)). An output signal representing the oppositeseries of data values (00010010) is shown in broken lines in FIG. 1B;this is the same as the signal shown in solid lines, but inverted.

FIG. 1C depicts the operation of a receiver, which is receiving thesignal of FIG. 1B with no distortion or noise. The receiver determineswhether the value of the signal parameter is above or below a receiverthreshold H_(R) at a sampling time T_(S). If the value of the signalparameter is above H_(R), the receiver determines that a digital value 1was received; if the signal parameter is below H_(R), the receiverdetermines that a digital 0 was received. Ideally, T_(S) is exactlycentered in the bit interval T_(B) and H_(R) is exactly equal to H_(T).However, as indicated in FIG. 1C, these conditions may not be fulfilledexactly. Because the signal parameter rises and falls abruptly andalways reaches the full discrete value V₁ or V₂ during each datainterval, the receiver will still detect the correct digital values evenif H_(R) is offset somewhat from H_(T) and T_(S) is offset from theexact center of the data interval. The shaded area in FIG. 1C depictsthe margins for errors in centering T_(S) and in aligning H_(R) withH_(T). Because the shaded area can have a shape resembling an eye, sucha diagram is commonly referred to as a “data eye”.

FIG. 1D depicts the signals of FIG. 1B after transmission through asignal channel which attenuates high frequency components of the signal,at frequencies above the bit toggle frequency f of the signal, withoutsubstantially attenuating components below the bit toggle frequency f.Here again, the solid-line graph depicts the signal representing theseries of digital values 11101101 and the broken-line graph depicts thesignal representing the opposite series of digital values. Referring tothe solid-line graph, because the high-frequency components of thesignal are attenuated, the signal parameter decreases gradually from V₁towards V₂ after commencement of the fourth bit interval. The signalparameter never reaches V₂ during the bit interval, but instead onlyreaches an intermediate value V_(I) at the end of this bit interval.Note that the signal parameter barely crosses the transmitter thresholdH_(T). During the next bit interval T_(b5), the signal parameterincreases back from the intermediate V_(I) value toward V₁. Because thelow-frequency components of the signal below the bit toggle frequency fare not appreciably attenuated by the signal path, the signal parametermay reach its full value V₁ during T_(b5) or T_(b6). Thus, the signalstarts again from the full value V₁ during the following bit intervalT_(b7). Here again, the broken-line graph shows the opposite pattern,with the signal parameter increasing gradually from V₂ to anintermediate value V_(I′) during the bit interval T_(b4) where thesignal represents a digital 1 after a series of digital 0's.

As shown in the eye diagram of FIG. 1E, the height of the data eye forthe receiver is (V_(I′)-V_(I)) rather than the full swing S or V₁-V₂.Stated another way, a high-frequency attenuation factor of A reduces theheight of the eye opening from S to S(2A−1) with the eye opening orshaded area completely disappearing at A<0.5. The height of the eyeopening is the amount of effective signal swing available to tolerateoffset of the receiver threshold H_(R) from the transmitter thresholdH_(T) and other sources of noise such as crosstalk, reflections ofprevious bits, and coupled supply noise, any or all of which may causethe signal parameter as received to vary from the values shown in thegraphs. Moreover, the width or temporal extent of the eye is alsoreduced to a time T_(E) which is less than the full bit interval T_(B).This effect causes greater sensitivity to misalignment between thesampling time T_(S) and the center of the data eye, and hence causesgreater sensitivity to skew and jitter in the signal or sampling clock.Moreover, the effects of signal distortion on the waveform may introducenoise into the timing loop used by the receiver to control the samplingtime T_(S).

FIG. 2 shows one embodiment of an equalizing transmitter 10. Thetransmitter 10 can be used, by way of example, as part of an integratedcircuit device coupled to a channel or signal path where the output ofthe transmitter 10 is sent over the channel or signal path 50 to areceiver (not shown). The transmitter 10 provides channel equalizationby modifying a square-wave-like signal to form a transmitted signal sothat a concatenation of the transmitted signal and the frequencyresponse of the channel provide a substantially square-wave-like signalat the receiver.

In the depicted embodiment, the transmitter 10 includes a first driver12 and a second driver 14 and receives a digital input signal DATAIN.The first driver 12 outputs a first output data signal DATAOUT1representing the digital input signal DATAIN. The second driver 14includes a high pass filter 16 and generates a second output signalDATAOUT2 representing a high pass filtered version of the input signalDATAIN. The output signals DATAOUT1, DATAOUT2 are combined to produce aresultant channel equalized signal DATAOUT for transmission over thechannel 50.

FIG. 3 shows a circuit diagram of an embodiment of the transmitter 10 ofFIG. 2. The transmitter 10 in the embodiment shown in FIG. 3 is adifferential signaling mode transmitter that receives input signalDATAIN and generates the equalized signal DATAOUT in the form of adifferential output signal at a pair 21 of differential output terminalsLINE+, LINE− for transmission over the channel 50, which in this case isa differential channel having at least two conductors 51 and 52. In thisembodiment, the transmitter 10 is a current mode transmitter with thefirst driver 12 being a current mode driver having a substantiallyconstant current source ISOURCE that causes signaling current to flowfrom power supply terminal Vdd to power supply terminal GROUND. In oneembodiment, the first driver 12 is configured as a differential driverwith a set of metal oxide semiconductor (MOS) transistors 32, 34 havingtheir respective drain terminals coupled to the power supply terminalVDD through termination resistors 28, 30, respectively. The drainterminals of transistors 32, 34 are also connected to respectivedifferential output terminals LINE+ and LINE−. The gate terminals oftransistors 32 and 34 are driven by differential input data signalsDATAIN+ and DATAIN−, respectively.

In one embodiment, as shown in FIG. 3A, DATAIN+ and DATAIN− are derivedfrom two independent data streams D0 and D1 using, for example, amultiplexer 24, which clocks the bits in the data streams D0 and D1 ontoDATAIN+ and DATAIN−. DATAIN+ may be the non-inverting output of themultiplexer 24 and DATAIN− may be the inverting output of themultiplexer, and vice versa. The multiplexer 24 may be a 2:1 multiplexerwith two data inputs and a selector input coupled to a clock signal CLKto select between the two inputs. In one embodiment, the clock signalCLK is a periodic signal and the two independent sequences of digitalbits D0, D1 are received at respectively ones of the two data inputs ofthe multiplexer 24 in parallel. When the clock signal CLK is at a highvalue, a digital bit D0 in a first one of the two sequences is selectedand when the clock signal is a low value, a digital bit D1 in a secondone of the two sequences is selected. Thus, the non-inverting outputDATAIN+ supplies first the first bit, or digital value (0 or 1) ofsequence D0, then the first bit of sequence D1, then the second bit ofD0, then the second bit of D1, and so forth of input signal DATAIN inserial format. Simultaneously the inverting output DATAIN− supplies thecomplement of the bits, or digital values, supplied by DATAIN+.

In one embodiment, the second driver 14 is a pulse mode driver having afilter 16 that receives signals DATAIN− and DATAIN+ and generates a pairof outputs 15 a and 15 b having a rapid rise (or fall) followed by anexponential decay toward a zero signal level, herein referred to as a“spike”, corresponding to each rising (or falling) edges of signalsDATAIN− and DATAIN+, respectively. The filter 16 is shown to include aplurality of high-pass continuous-time passive filter networks 16 a, 16b. The filter network 16 a includes a series resistor-capacitor (RC)network comprising a filter resistor 20 a and a filter capacitor 22 a,and an inverter 18 a. Likewise, the filter network 16 b includes aseries resistor-capacitor (RC) network comprising a filter resistor 20 band a filter capacitor 22 b, and an inverter 18 b. The values of theresistors and capacitors in the RC networks 16 a and 16 b can beselected to set a cutoff frequency and gain of the filter 16. The inputsof filters 16 a, 16 b are connected to the inverting and non-invertingoutputs of the multiplexer 24, respectively, and the outputs 15 a and 15b of the filters 16 a and 16 b, respectively, are connected todifferential output terminals LINE+, LINE−, respectively. The outputs 15a and 15 b of the filters 16 a and 16 b, along with the output of thetransistors 32, 34, are combined at respective first and second nodesSUM1 and SUM2 to provide output signal DATAOUT over differentialterminals LINE+, LINE−, respectively.

The filter 16 introduces a zero into the transfer function of thetransmitter 10 because of the high pass filter characteristics of thefilter. The zero of the filter 16 offsets the pole of the low passfilter characteristics of the attenuation of the signal channel 50. Acombined transfer function H(Filter) of the filter 16 and transmitter 10may be represented by:

${H({Filter})} = \frac{1}{\left( {R + \frac{RO}{4}} \right)C^{\prime}}$where R represents the filter resistance, C represents the filtercapacitance and RO represents the differential impedance of the channel50. In this embodiment, the filter resistance R is related to filterresistor 20 a and/or 20 b in addition to the output resistance ofinverters 18 a and/or 18 b, the capacitance C is related to filtercapacitor 22 a and/or 22 b and channel impedance RO is related to theimpedance of the channel 50. For a system intended to send an outputsignal at a bit toggle frequency of about 5 GHz, a typical value for thefilter resistor 20 a is about 1000 ohms, the filter capacitor 22 a isabout 30 femtofarads, and the channel impedance may be about 100 ohms,which is about the same value as twice the resistance of the terminationresistor 28 or 30. The impedance of the RC network of filter 16 isrelatively high compared to the impedance of channel 50 so that theoutput termination impedance of transmitter 10 is determined mainly byresistors 28 and 30 and is therefore nearly resistive, providing ahigh-quality termination for channel 50.

FIGS. 4 and 5 illustrate an operation of the transmitter 10 of FIGS. 2and 3. FIG. 4 depicts current flow through the first driver 12 of thetransmitter 10 assuming DATAIN+ is at a high value 1 and DATAIN− is at alow value 0, which is the complement of the high value 1. It is alsoassumed that the output of the transmitter 10 is connected to a channel50 having an impedance represented by a resistor 36. For example, thechannel impedance 36 may be about 100 ohms and the impedance of each oftermination resistors 28, 30 may be about 50 ohms.

In the state depicted in FIG. 4, transistor 34 is ON (in a conductivestate) in response to the high value of DATAIN+ and transistor 32 is OFF(in a non-conductive state) in response to the low value of DATAIN−. Inthis condition, a current I flows through the source and drain terminalsof transistor 34 to the constant current source ISOURCE. Transistor 32,in its non-conductive state, prevents current flow through its sourceand drain terminals. In this condition, a current path from VDD toISOURCE through resistor 30 has a total impedance substantially equal tothe impedance of termination resistor 30, which, in one example, may beabout 50 ohms. Another current path from VDD to ISOURCE has a totalimpedance equal to the sum of the impedance of termination resistor 28and the channel impedance 36, which sum, in one example, is about 150ohms. As a result, the current flowing through resistor 28 and channelimpedance 36 is about I(¼) and the current flowing through resistor 30is about I(¾). The current I(¼), along with the current I(¾), is summedat summing junction SUM2 to provide a total current of I which flowsthrough transistor 34. Although not shown, the first driver 12 works ina complementary manner when the transistor 34 is driven by a low valueand the transistor 32 is driven by a high value. That is, the transistor34 turns OFF (non-conductive state) and the transistor 32 turns ON(conductive) which allows current I(¼) to flow through channel impedance36 but in the opposite direction to that of the above example. Thecurrent I is shown to be a steady state signal.

FIG. 5 illustrates current flow through the second driver 14 of thetransmitter 10 according to one embodiment of the present application.In one embodiment, the second driver 14 is a pulse mode driver thatfunctions in response to transitions in the input signal DATAIN and notto a steady state high or low value of the input signal. In FIG. 5, itis assumed that the input DATAIN+ to inverter 18 b is making a positive(from low to high) signal transition and the input DATAIN− to inverter18 a is making a negative (from high to low) signal transition.

The positive signal transition on the inverter 18 b causes it togenerate a negative voltage transition on its output. In a complementarymanner, the negative signal transition on the input to the inverter 18 acauses it to generate a positive signal voltage transition on itsoutput. The positive voltage transition on the output of the inverter 18a causes a current I₁ to flow through the RC network of filter 16 a suchthat a positive current spike signal is generated at the summingjunction SUM1. The negative voltage transition on the output of theinverter 18 b causes current I₁ to flow through the RC network of filter16 b such that a negative current spike is generated at the summingjunction SUM2. The current signal I₁ at the summing junction SUM1 isdistributed through two current paths: an amount of current I₁ (½) flowsthrough the channel 36 and an amount of current I₁ (½) flows throughtermination resistor 28 to the power supply source VDD. The spikecurrent signal I₁ at the summing junction SUM2 is the sum of current I₁(½) flowing through the channel 36 and current I₁ (½) flowing throughtermination resistor 30 from the power supply source VDD. Thus, thevoltage transitions on the output of the inverters 18 a, 18 b associatedwith the filters 16 a, 16 b cause currents to flow through each RCnetwork of the filters and onto the channel 50. For an oppositetransition in the digital signal DATAIN, filters 16 a, 16 b operate in acomplementary way to provide an oppositely-directed current pulse orspike on the channel 50.

The second driver 14 works in combination with the first driver 12 tooverdrive each signal transition by providing an additional boost to thesignal provided by the first driver 12 at or immediately after eachtransition of the input signal DATAIN. The additional boost may beachieved by using the energy from the filter capacitors which functionas charge pumps to drive additional current to the channel. Both thefirst driver 12 and the second driver 14 may be referenced to the samepower supply voltage VDD so both output currents used to drive thechannel are proportional to the supply voltage. The magnitude of theadditional current provided by the second driver 14 may be representedby the following:|I ₁ |=VDD/(R+RO/4),where VDD is the supply voltage, R represents the filter resistance andRO the channel impedance. In an embodiment, the magnitude of theadditional current I₁ is about 1 mA based on a supply voltage VDD ofabout 1 V, the filter resistor R of about 1000 ohms and the channelimpedance RO of about 100 ohms. In addition, the second driver 14consumes relatively little power because it operates only upontransitions in the input signal DATAIN, and consumes nearly zero powerwhen the digital input signal DATAIN does not change value.

FIG. 6 shows an operation of the transmitter 10 in response to an inputsignal DATAIN which in this example includes a series of bits havingvalues 0010110 corresponding to a plurality of bit intervals t₀ throught₆. The signal DATAOUT1 is generated by the first driver 12, whereas thesignal DATAOUT2 is generated by the second driver 14, and the outputsignal DATAOUT is the combination of signals DATAOUT1 and DATAOUT2.DATAOUT is applied to the signal path 50 as the output signal of thetransmitter 10 in response to the input signal DATAIN. Signals DATAOUT1,DATAOUT2 and DATAOUT are depicted in FIG. 6 as currents in the signalpath 50, with current in the direction from LINE+ to LINE− (FIG. 4)generating a positive voltage between LINE+ and LINE− and current in thedirection from LINE− to LINE+ (FIG. 4) generating a negative voltage. Atransmitter threshold or central value H_(t) is set at about zero signallevel (e.g., no current flowing in the channel, and no voltage acrosspins LINE− and LINE+).

During the first two bit intervals t₀ and t₁, the digital value beingtransmitted is 0, and the signal parameter, i.e., the current, of signalDATAOUT1 remains at a constant, discrete value I₂ denoting the digitalvalue 0. I₂, in this case, may be a negative current, about −1 mA, forexample. During these bit intervals, there is no transition in digitalvalues or the discrete values of the signal parameter, and the seconddriver 14 remains inactive, resulting in signal DATAOUT2 having a signalparameter or current of zero. Thus, the output signal DATAOUT has aconstant current or signal parameter at the value I₂.

In the next bit interval t₂, the digital value 1 is supplied to drivers12 and 14. The first driver 12 provides the signal DATAOUT1 with asignal parameter or current having a different discrete value I₁denoting the digital value 1, which, in this case, is a positive currentof about +1 mA, for example and remains constant during the bitinterval. In response to the transition in the digital value between bitintervals t₁ and t₂, the second driver 14 generates signal DATAOUT2 as acurrent pulse or spike that rises rapidly to value I3, beginning at thecommencement of the bit interval t₂, and decays toward a steady statevalue (e.g., zero magnitude) during that bit interval. The output signalDATAOUT during interval t₂ is the sum of the discrete value I₁ and thespike, so has a value I₁+I₃ at the beginning of the bit interval. Thetransition in this case is a positive transition or has a positive sensebecause it corresponds to a positive change in the digital value, from 0in time interval t₁ to 1 in time interval t₂, or an increase in thesignal parameter supplied by driver 12, from I₁ in time interval t₁ toI₂ in time interval t₂ succeeding the time interval t₁. As a result, thespike in the signal parameter or current supplied by driver 14 also hasa positive value. Stated another way, the spike used during thepost-transition interval t₂ has the same sense (positive) as thecorresponding transition.

A negative transition occurs between time interval t₂ and time intervalt₃, corresponding to a negative change in digital values (from 1 to 0).In the next bit interval t₃, a 0 digital value is supplied to thedrivers. The first driver 12 generates DATAOUT1 with the negativecurrent or discrete value I₂ denoting a 0 digital value, whereas thesecond driver 14 generates a negative pulse or spike in DATAOUT2 at thecommencement of interval t₃, which drops rapidly after the transitionbetween intervals t₂ and t₃ to a value of I₄ and decays gradually upwardtoward zero during the interval t₃. Again, spike or current pulseoccurring during the post-transition interval has the same sense as thetransition itself, so that the current driving into the channel is I₂+I₄at the beginning of the bit interval.

In the next interval t₄, a digital value of 1 is supplied to thedrivers. As in interval t₂, driver 12 supplies signal DATAOUT1 withdiscrete value I₁ denoting digital value 1, and driver 14 supplies apositive current pulse or spike with initial amplitude I₃ in response tothe transition between digital values. Here again, the pulse or spikehas the same sense as the transition, and the initial amplitude at thebeginning of the bit interval is I₁+I₃.

In the next interval t₅ of the same clock cycle, the digital value 1 isagain supplied to the drivers. The output DATAOUT1 of the first driver12 remains at discrete value I₁. Because there is no transition betweentwo successive digital values in DATAIN, the second driver 14 does notgenerate a pulse or spike output.

At the commencement of the next interval t₆, the digital value DATAINtransitions from 1 to 0. During interval t₆, driver 12 supplies DATAOUT1with the discrete value I₂ denoting zero, whereas driver 14 generatesDATAOUT2 as a negative pulse or spike which decays gradually upwardtoward zero.

In summary, DATAOUT1 is mostly a replicate of the sequence of digitalinput values DATAIN, in that DATAOUT1 includes a series of discretevalues (e.g., I₁ and I₂) of an output signal parameter (e.g., current)denoting the digital input values. DATAOUT2 includes spikes duringpost-transition data intervals, the spike during each post-transitiondata interval has the same sense as the transition between discretevalues at the commencement of such data interval. As discussed above,the output signal DATAOUT supplied to the signal path is a combination(e.g., a sum or superposition) of DATAOUT1 and DATAOUT2.

Thus, the equalizing transmitter 10 is operative to output a signalDATAOUT that is a combination of two signal components, a first signalcomponent DATAOUT1 having one of a plurality of discrete values for eachone of a series of data intervals (e.g., t₀-t₆), and a second signalcomponent DATAOUT2 that resembles a high-pass filtered version of thefirst signal component. In the first signal component, a plurality ofdiscrete values represent respective ones of a series of digital values.In the second signal, a spike or pulse corresponds to each transition inthe first signal component, which transition corresponds to a changefrom a first discrete value corresponding to a first data interval to asecond, different discrete value corresponding to a second data intervalsucceeding the first data interval. The spike or pulse has a same senseas the transition and includes a sharp change from a steady state valueof the second signal component followed by an exponential decay towardthe steady state value.

The inclusion of the spikes or pulses materially boosts thehigh-frequency content of DATAOUT, at frequencies about the same orabove the bit toggle frequency ½T_(B), as compared with DATAOUT1. Thereare two distinct parameters of filter 16 that can be adjustedindependently to help cancel ISI in the communications channel. Thefirst parameter is the “gain” G of the filter. Referring to FIG. 6, thefilter gain G=I₃/I₁=I₄/I₂. The second parameter is the cutoff frequencyof the filter, defined previously.

Referring to FIGS. 3, 4, and 5, the gain of the filter 16 can beadjusted by varying the relative currents produced in data transmitters12 and 14. For example the inverters 18 a and 18 b may obtain theirsupply voltage from the same power supply terminals, VDD and GROUND, astransmitter 12. In this case, the amplitude of the output current fromtransmitter 14 is about equal to VDD/(R+R0/4), and the current deliveredfrom transmitter 14 to the channel is |I₃|=|I₄|=0.5*VDD/(R+R0/4). Thecurrent in the ISOURCE constant current source in transmitter 12 can bemade proportional to VDD/R0, and the current delivered to the channelfrom transmitter 12 is therefore |I₁|=|I₂|=K*VDD/(4*R0), where K is aproportionality constant established during the design process or duringoperation of transmitter 10. The filter gain isG=|I₃|/|I₁|=2*R0/(K*(R+R0/4)). Therefore the filter gain can be adjustedby changing the current in ISOURCE, by varying K, or by changing thevalue of resistor R. Alternatively, inverters 18 a and 18 b may bepowered from a variable power supply other than VDD, and the filter gainmay be changed by adjusting the voltage on this power supply. Thesealternatives are provided by way of example only, and any other meansmay be used to vary the filter gain that effectively changes theresistance or source voltage in filter 16.

The second filter parameter, the filter cutoff frequency, may likewisebe varied over a range of values. The cutoff frequency may be variedfrom frequencies greater than the bit toggle frequency to frequenciesbelow the bit toggle frequency. However, when the cutoff frequency offilter 16 is set below about half the bit toggle frequency, the methodbecomes less effective because the signal from transmitter 14 does notreturn substantially to zero during a single bit interval. The filterfrequency may be varied by changing the value of the capacitors 22 a and22 b, leaving the values of resistors 20 a and 20 b fixed, or it may bevaried by leaving the capacitors fixed and varying the resistance of theresistors. The latter method also changes the gain of the filter, so isgenerally less desirable.

FIG. 7A shows the frequency response H(Channel) of an example channelhaving two poles at the bit toggle frequency of 5 GHz. FIG. 7B shows thefrequency response H(Filter) of the transmitter equalizer, thecombination of transmitters 12 and 14. H(Filter) in this example has ahighpass cutoff, or zero, frequency equal to the bit toggle frequency,and the gain of the filter is set to G=2.0. FIG. 7C shows thecombination response H(Filter)*H(Channel) of filter equalizationH(Filter) of FIG. 7B and channel attenuation H(Channel) of FIG. 7A. Asseen in FIG. 7C, the high pass filter function H(Filter) substantiallycancels the low pass (high-frequency attenuation) of the channelH(Channel) providing a fairly flat response over the frequency band ofinterest from near DC to about half the bit toggle frequency, and modest(3 dB) attenuation at the bit toggle frequency. As explained above, thecutoff frequency of the filter can be adjusted by varying the value ofthe RC network of the filter 16. In this example, the cutoff frequencyof the high-pass filter 16 is set at the bit toggle frequency of theinput signal, assumed to be 5 GHz.

FIG. 7D shows a data eye diagram for a pseudo-random bit sequencetransmitted through the channel, whose frequency response is shown inFIG. 7A, but without equalization. A data eye diagram is a graph inwhich a series of 2-unit-interval-long time sequences of the data streammeasured at the end of the channel are overlapped and superimposed alongthe time axis. The time axis is expressed in terms of bit intervals orunit intervals, with 0 UI at the nominal center of the eye. FIG. 7Dshows that the eye is “closed”, that is the height or amplitude of thesignal has been reduced by about 0.3 due to inter-symbol interference inthe channel. Further, the horizontal extent of the eye along the timeaxis has also been reduced by about 0.5 due to inter-symbol interferencein the channel.

FIG. 7E shows a data eye diagram for the same pseudo-random bit sequenceas shown in FIG. 7D, but using the transmitter equalizer, whosefrequency response is shown in FIG. 7B, to drive the channel. Theequalization filter has opened the eye to about 0.9 in its verticalextent and to nearly 1.0 in the horizontal extent.

FIG. 8A shows another example of a channel response H(Channel), thisexample having a single pole at 1 GHz, or one-fifth of the bit togglefrequency. Attenuation at the bit toggle frequency for this channel isabout 0.2. FIG. 8B shows the filter frequency response H(Filter), wherethe filter cutoff frequency or pole frequency is still set at the bittoggle frequency, and the filter gain is set at G=4.2. FIG. 8C shows thecombined response H(Filter)*H(Channel) for the transmitter-equalizedchannel. As in the example of FIGS. 7A through 7C, the combined responseis essentially flat to about half of the bit toggle frequency and showsmodest (3 dB) attenuation at the bit toggle frequency.

FIG. 8D shows a data eye diagram for a pseudo-random bit sequencetransmitted through the channel with frequency attenuation shown in FIG.8A. This eye is completely closed, and it would be impossible to recoverthe transmitted data passing through this example channel. FIG. 8E showsa data eye diagram for the equalized channel, whose frequency responseis shown in FIG. 8C. This eye is again opened to 0.9 in vertical extentand 1.0 in horizontal extent by the transmitter equalizer.

FIG. 9A shows yet another example channel response H(Channel), this onecomprising two poles located at 2.5 GHz, half the bit toggle frequency.Attenuation at the bit toggle frequency is about 0.15. FIG. 9B shows thefrequency response of the transmitter equalizer H(Filter) with cutofffrequency again set at the bit toggle frequency, and gain set at G=4.5.FIG. 9C shows the combined frequency response of channel and filterH(Filter)*H(Channel). The equalized response is flat to about ⅕ of thebit toggle frequency, and attenuation at the bit toggle frequency isabout 0.6.

FIG. 9D shows a data eye diagram for a pseudo-random bit sequencetransmitted through the channel with frequency attenuation shown in FIG.9A. This eye is completely closed. FIG. 9E shows a data eye diagram forthe equalized channel, whose frequency response is shown in FIG. 9C.This data eye, while not fully open in the vertical extent, is opened toabout 0.7 of the signal parameter and would be easily detected by areceiver at the end of the channel. The horizontal extent of this eye isopened to essentially 1.0.

Considered qualitatively, the presence of the spikes associated withdata transitions increases the amplitude of the signal after atransition between different discrete values of the signal parameter. Asdiscussed above with reference to FIGS. 1A-1E, and as shown in FIGS. 7A,8A, and 9A, the signal path tends to attenuate high-frequency componentsto a greater degree than low frequency components, and thus tends toconvert a sudden transition into a more gradual transition. The presenceof the spikes compensates for this characteristic of the signal path.After attenuation in the signal path, the waveform of DATAOUT mayapproximate the waveform of DATAOUT1, with approximately squaretransitions between different values of the signal parameter. Asexplained above in connection with FIG. 1C, such a waveform provides agood data “eye” for operation of the receiver.

Again, the spikes or current pulses are provided during post-transitiondata intervals, following transitions between digital values. Thepulse-mode circuits used in the second driver 14 draw essentially nopower between transitions. This is in marked contrast to a differentialcurrent-steering driver using a current source such as the first driver12. A differential current-steering driver continues to draw currentthrough the current source even when it is producing a zero differentialsignal; in that condition, equal current flows pass through bothtransistors. The pulse-mode circuits do not suffer from this drawback.Additionally, the data transmitter 14 need only provide sufficientamplitude to overcome DC losses in the channel, and fixed sources ofnoise, such as thermal noise and input offset in the receiver. For allof these reasons, the equalizer 10 uses power efficiently, in that thepower incorporated in the spikes or pulses can be relatively small, andthe power in the no-equalizing transmitter 12 can be reduced to theminimum required by the channel and receiver. Stated another way, for agiven power consumption, the signal-to-noise ratio of the signal can behigher than would be in the case with a less efficient equalizer, suchas a conventional transmitter equalizer.

The equalization arrangement discussed herein can be incorporatedreadily in monolithic integrated circuits. For example, as depicted inFIG. 10, a semiconductor chip 150 has several equalizing transmitters 10a-10 d formed within the chip, each equalizing transmitter 10 a-10 dincluding an equalizer 10 as discussed herein. The data input of eachtransmitter 10 a-10 d is connected to an operative circuit 151 withinthe chip 150. The operative circuit 151 may include one or more centralprocessing units, memory circuits, bridge circuits, or the like. Theoperative circuit may be a circuit which requires relatively high-speeddata communication as, for example, at data rates at or above 300Mbits/sec, or at or above about 1 Gbit/sec as, for example, at about 10GBit/sec. Equalizing transmitters 10 b, 10 c and 10 d have their outputconnections 21 coupled to contacts 121 of the chip so that the outputconnections can be electrically connected to a signal path external tothe chip. For ease of illustration, only one contact 121 is depicted inFIG. 10 associated with each of these transmitters. However, where theoutput connection includes a pair of connections as discussed above, apair of external contacts is provided for each transmitter. Transmitter10 a has its input connection coupled to a part 151′ of the operativecircuit in the chip, and has its output connections 21 coupled to asignal path 50 a extending within the chip to a receiver 152 also formedwithin chip 150. The output of the receiver is coupled to another part151″ of the operative circuit. Thus, transmitter 10 a is used forcommunication within the chip. This arrangement can be used, forexample, for communication between widely-separated regions of the chip.Any number of transmitters 10 can be formed within the chip, and can beconfigured for intra-chip or off-chip communication as desired.

Chip 151 can be used as an element of a larger system 153. The systemmay include a housing 154, one or more additional electronic components155 such as additional semiconductor chips or modules, disposed withinthe housing, and one or more internal signal paths 156 extending withinthe housing. For example, the off-chip connections 121 of transmitters10 b and 10 c on chip 151 are connected by the internal signal paths 156to receivers 157 incorporated in additional component 155. To providebidirectional communication, the additional component 155 may have oneor more transmitters, such as transmitter 10 e connected by otherinternal signal paths to a receiver 158 incorporated within the chip,such receiver being connected to the operative circuit 151 of the chip.Transmitter 10 e may be a transmitter as discussed herein. Pluraltransmitters, such as transmitters 10 b and 10 c, can be used to sendmultiple data streams in parallel. Chip 151 and additional component 155may be part of a system which uses digital information. Merely by way ofexample, the operative circuit of chip 151 may include a centralprocessing unit or CPU as commonly employed in a digital computer,whereas the additional component 155 may be a bridge chip commonlyemployed as an intermediary between the CPU and other chips. Also, chip151 may be a memory chip and the additional component may be aprocessing unit such as a CPU or another chip which serves to connectthe memory with the CPU for interchange of information between thememory and the CPU. For example, transmitters as discussed herein can beincorporated in the communication path between a processor such as a CPUand a cache memory. In other embodiments, chip 151 or additionalcomponent 155 may be an element of a peripheral device such as a datainput or output device controller or the like.

Transmitter 10 e on chip 151 is connected through its off-chipconnection 121 to an external signal path 158 which extends outside ofthe device housing 154 and which is connected to a receiver in anexternal element. External element 159 optionally may have one or morefurther transmitters 10 f connected to chip 151 to provide bidirectionalcommunication.

Of course, although only a few internal and external signal paths areshown, any number of such paths can be used. The internal signal paths156 and external signal path 158 may be baseband signal paths, i.e.,signal paths without modulation or demodulation. Most commonly, thesignal paths may include conductors or pairs of conductors as, forexample, conductors in printed circuit boards, cables or the like. Suchbaseband conductors are often used for signal transmission overrelatively short distances as, for example, about 10 meters or less.Merely by way of example, transmitters as discussed herein may be usedas elements of point-to-point connections according to protocols such asPCI Express, Serial ATA and other protocols. Also, the transmitters asdiscussed herein can be used with bus connections, i.e., arrangements inwhich the same signal is sent to plural devices connected to the sameconductors.

In one embodiment, as shown in FIG. 11, each of filters or filternetworks 16, 16 a, and 16 b may include a set of filters 116 a, 116 b, .. . , 116 n configurable to provide a filter transfer function H(Filter)having an adjustable gain and/or cutoff frequency. The filters 116 a-116n include RC networks comprising filter resistors 120 a-120 n and filtercapacitors 122 a-122 n, respectively, similar to the resistor-capacitornetwork of filter 16 a discussed above with reference to FIG. 3. Thefilters 116 a-116 n are coupled to each other in parallel but areenabled based on the state of enable signals En [0]-En [n],respectively. The filters 116 a-116 n include tristate inverters 118a-118 n controlled by buffers 115 a-115 n via enable signalsEn[0]-En[n], respectively. For example, in filter 116 a, when the signalEn[0] is set to a high value, the buffer 115 a enables the tristateinverter 118 a which connects the filter 116 a to other enabled filters.In contrast, when the enable signal En[0] is set to a low value, thebuffer 115 a disables the tristate inverter 118 a which disconnects thefilter 116 a from other filters.

The characteristics of the filters 116 a-116 n can be selected toprovide a filter transfer function H(Filter) with a particular gain andfrequency cutoff. For example, the value of the resistors 120 a-120 nand capacitors 122 a-122 n can be set to the same value. In this case,the gain G of the transfer function of the filter H(Filter) increases asthe number of filters is enabled. In another example, the value of theresistors 120 a-120 n and capacitors 122 a-122 n can be set to differentvalues set to provide zeros at different cutoff frequencies. In thiscase, the order of the transfer function of the filter H(filter)increases as the number of filters is enabled. Alternatively, acombination of the above approaches can be employed to provide a filterwith increased amplitude and different cutoff frequencies.

The enable signals En[0]-En[n] can be programmed to selectively enablethe filters 116 a-116 n in accordance with various algorithms ortechniques to provide a filter transfer function H(Filter) with aparticular gain and frequency characteristic. For example, during designof a system which incorporates a chip having a transmitter equalizer,the transfer function of the filter H(Filter) can be calculated tooffset the frequency attenuation of a channel H(Channel) which is to becoupled to the output of the transmitter. Based on this information, thecombination of high and low enable signals En[0]-En[n] can be determinedto provide a particular transfer function H(Filter). An internal circuitwithin the chip (not shown) or an external circuit can be programmed tosupply the selected combination of enable signals. In a furtherembodiment, a receiver connected to a transmitter can evaluate channelconditions at its end and determine coefficients representing suchconditions. The receiver can send the coefficients to the transmittervia a separate channel (back channel) which can be used by thetransmitter to program the enable signals. This technique allows thetransmitter to adjust channel equalization dynamically based on changingchannel conditions. Suitable algorithms for determining the equalizationcoefficients include both edge-based algorithms and amplitude-basedalgorithms.

To implement an amplitude-based algorithm, for example, the receiver canbe configured during a startup sequence to measure the voltage amplitudeof the eye at the center (maximum amplitude point) of the eye, andacquire the amplitude averaged over a large number of data bits in aseries of random or pseudo-random bits transmitted from transmitter toreceiver. If this average eye height or amplitude is acquired for eachof a number of settings of the transmitter equalizer, an optimum settingcan readily be determined by taking the setting that yields the highestaverage amplitude. Alternatively, the receiver can be equipped with anextra sampler, not used for receiving data from the transmitter, thatcan sample the amplitude of the data eye continuously during normal datatransmissions. This ‘extra’ data sampler's measurement of averageamplitude over many bits can be used to drive the equalization valuetoward an optimum value without the need to interrupt normal datatransmission. This type of equalization adjustment can accommodatechanges in H(Channel) that may result from, for example, temperaturechanges to the system that embeds the channel.

To implement an edge-based algorithm for adjusting the equalizationsetting in the transmitter, the receiver may be equipped with an“Alexander” style phase detector, in which samplers are provided tosample the data stream in the center of each data eye (data samplers)and other samplers are provided to sample the data stream at the “edges”or data transitions between successive data bits (edge samplers). Aseries of edge and data samples is taken, such that each successive edgeand data sample represents the digital value of the received signalevery half bit interval, denoted E(0), D(0), E(1), D(1), E(2), . . .where D(1) is the data sample taken one bit interval after D(0), E(1) isthe edge sample taken one bit interval after E(0), and so forth. If D(1)differs from D(0) it can be inferred that there is a data transitionbetween these two data samples. If D(1) does not equal D(0) andE(1)=D(1), it can be inferred that the samples were taken “late” andthat the sampling clock within the receiver should be retarded in timeto sample the data bits at the optimum point (highest amplitude) at thecenter of the data eye. If however D(1) does not equal D(0) andE(1)=D(0), it can be inferred that the samples were taken “early” andthat the sampling clock should be advanced in time. These “early” and“late” indications can be further used to determine if the equalizationsetting in the transmitter is adjusted optimally. For example, assumethat the number of “early” and “late” indications averaged over a largenumber of random data bits is the same, indicating that the samplingclocks in the receiver are optimally placed. If the received data is nowsearched for a pattern having a large number of 0's followed by a single1 followed by at least one 0 and the early/late indication is examinedfor the transition from 0 to 1 before and the early/late indication forthe transition from 1 back to 0, then if the early/late indication forthe 0 to 1 transition is consistently “early” while the indication forthe 1 to 0 transition is consistently “late”, it can be inferred thatthe channel is under-equalized and that the gain G of the equalizationfilter in the transmitter needs to be increased. Similarly if theearly/late indication for the 0 to 1 transition is consistently “late”while the indication for the 1 to 0 transition is consistently “early”,it can be inferred that the channel is over-equalized and that the gainG of the equalization filter in the transmitter should be reduced. Ineither case, this information can be sent back to the transmitter over aback channel so that the equalizer can be adjusted appropriately. Notefurther that a data pattern having a large number of 1's followed by asingle 0 followed by at least a single 1 is equivalent, and itsearly/late indications may be analyzed in exactly the same way. Further,the correlation of early/late indications with other patterns may beused to drive toward the correct equalization setting.

Further, these previously described examples of an amplitude-based andan edge-based equalization adaptation algorithm are merely illustrativeexamples.

The second driver 14 discussed above with reference to FIG. 3 uses apair of filters 16 a and 16 b to provide differential-mode operation. Toprovide similar differential-mode operation, the second driver mayinclude two sets of filters as shown in FIG. 3. The two sets of filtersmay be set to have the same transfer function.

Still other embodiments are contemplated. For example, the transmitterhas been described in the context of a current mode transmitter, butother modes are contemplated such as a voltage mode transmitter having aconstant voltage source. The transmitter has been explained in thecontext of a differential mode of operation but other modes arecontemplated such as a single-ended mode. Also, in the current-modeexamples discussed above, the transmitter threshold H_(T) is at zerocurrent. In other systems, the transmitter threshold may be at anon-zero value of the signal parameter such as current or voltage. Forexample, in a current-mode system, a current in one direction having avalue less than a threshold value may denote the digital value 0,whereas a current in the same direction having a value greater than thethreshold may denote the digital value 1. Likewise, in a voltage modesystem, a positive voltage less than the threshold voltage may denotedigital value 0 whereas a positive voltage greater than the thresholdvoltage may denote digital value 1.

In yet another embodiment the two transmitters of FIGS. 2-5 can bereplaced with a single transmitter as shown in FIG. 12. As in FIG. 2, amultiplexer 24 generates true and complement versions of a data signalto be transmitted on DATAIN+ and DATAIN−. DATAIN+ and DATAIN− driveinverters 201 a and 201 b respectively, which form the drivers for thistransmitter. The inverters 201 a and 201 b operate between power supplyterminals VDD and GROUND and produce inverted versions of their inputsignals on their outputs, the signals toggling between voltages of aboutVDD for logic “1” and GROUND for logic “0”. The outputs of inverters 201a and 201 b drive two networks 202 a and 202 b respectively, containingresistors 203 a, 203 b, 204 a, 204 b, and capacitors 205 a and 205 b.The upper half of these networks, in which resistor 202 a is coupled inseries with capacitor 205 a and resistor 202 b is coupled in series withcapacitor 205 b, are analogous to the high-pass filters in transmitter14 in FIGS. 2 and 3. The lower halves of the two networks, comprisingresistors 204 a and 204 b, are analogous to transmitter 12 in FIGS. 2and 3. These networks are high-pass filters whose H(Filter) at DC is >0.The outputs of the two networks drive the transmitter outputs LINE+ andLINE−, which drive channel 50 to a receiver (or receivers). A terminatorresistor 206 is coupled between the two outputs LINE+ and LINE−.

When DATAIN+=“1” and DATAIN−=“0”, then in the steady state (notransitions on the DATAIN signals) inverter 201 a drives current I1through resistor 204 a, terminator 205, and then through resistor 204 band into inverter 201 b. Currents I2 in the upper half of each ofnetworks 202 a and 202 b are zero in the steady state, and currentI0=I1. If inverters 201 a and 201 b are assumed to have internalresistance Ri, the voltage developed between LINE+ and LINE− isVn=Vdd*R0/2*1/(R0/2+2*(R1+Ri))When DATAIN+=“0” and DATAIN−=“1”, then in the steady state (notransitions), then the voltage between LINE+ and LINE− is −Vn.

Immediately following a positive data transition, current I2 is nolonger 0, since current flows in the upper halves of networks 202 a and202 b. During the initial rapid rise of the “spike”, the voltagedeveloped between LINE+ and LINE− isVe=Vdd*R0/2*1/(R0/2+2Ri+2(R*R1/(R+R1)))The “gain” of this equalizing filter isG=(Ve−Vn)/Vn orG=2*R1²/((R0/2+2*Ri)*(R+R1)+2*R*R1)And the gain may be adjusted by changing the ratio of resistances R1/R.For a given desired gain, for example, R can be found fromR=(2*R1² −R1*G*(R0/2+2Ri))/(R0/2+2(Ri+R1))In all other respects, transmitter 200 operates essentially the same astransmitter 10. The characteristics of the transmitter, such as the gainand frequency response of the filter, can be easily modified or adjustedusing digital techniques. For example, the networks 202 a, 202 b can beconfigured to be adjustable in a manner similar to the techniquesemployed in filters 116 a-116 n shown in FIG. 11 and described above.The embodiment described in FIG. 12 is merely one example of many waysto implement a transmitter with a single driver and a high-pass networkwith adjustable ratio of high-frequency gain to DC gain. Otherembodiments may employ current-mode or voltage-mode drivers anddifferential or single-ended signaling. In one embodiment, one or moreof the transmitters 200 can be incorporated in a monolithicsemiconductor chip.

The transistors of the transmitter are shown and described as MOStransistors but may alternatively be implemented using bipolartechnology or any other technology in which a signal-controlled currentflow may be achieved. The transmitter has been described in the contextof a binary signaling system but other signaling modes are contemplatedsuch as multi-level signaling. In multi-level signaling, the discretevalue of the signal parameter applied during each data interval isselected from N discrete values, where N is greater than 2. Thus, thediscrete value of the signal parameter may denote the combined values oftwo or more bits in a binary digital input signal or may denote a singlemultilevel digital value in a multilevel digital input signal. Theformat of the digital input signal has been described with reference toFIG. 3 and FIG. 12 in the context of a digital signal comprising twostreams of digital bits in parallel, but in practice the digital inputsignal may be supplied in serial form or as any number of bits inparallel. Also, the digital input signal may itself be pre-processed orencoded before it is supplied to the transmitter. For example, aninformation bearing signal may be encoded into a further digital signalincorporating a greater number of bits or multilevel digital values toprovide a pre-processed signal, and that preprocessed signal may besupplied as the digital input signal to the transmitting equalizer asdiscussed herein.

Although the application herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent application. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

What is claimed is:
 1. A transmitter comprising: a driver to generate afirst signal representing a digital input signal; and a passivehigh-pass network adapted to receive the digital input signal, thepassive high-pass network including circuit elements having valuesselected to generate a second signal having spikes corresponding to eachtransition in the digital input signal, each spike in the second signalhaving one of 1) a rapid rise from a zero signal level corresponding toa rising transition, and 2) a rapid fall from a zero signal levelcorresponding to a falling transition, followed by a gradual decay tothe zero signal level, the transmitter being configured to combine thefirst and the second signals to form an output signal.
 2. Thetransmitter of claim 1, wherein the passive high-pass network has atleast one of an adjustable cutoff frequency and an adjustable ratio of ahigh-frequency gain to a DC gain.
 3. The transmitter of claim 1, whereina frequency cutoff of the passive high-pass network is set to be aboutequal to a bit toggle frequency of the input signal.
 4. The transmitterof claim 1, wherein the passive high-pass network is a continuous-timehigh pass filter comprising a resistor-capacitor network.
 5. Thetransmitter of claim 1, wherein the transmitter is configured to becoupled to a channel having a frequency attenuation, and wherein thepassive high-pass network provides a filter transfer function to offsetthe frequency attenuation.
 6. The transmitter of claim 1, wherein thepassive high-pass network is a first passive high-pass network and theoutput signal is a first component of a differential output pair.
 7. Thetransmitter of claim 6, further comprising a second passive high-passnetwork adapted to receive the digital input signal and to generate athird signal having spikes corresponding to each transition in thedigital input signal, each spike in the second signal having one of 1) arapid rise from a zero signal level corresponding to a risingtransition, and 2) a rapid fall from a zero signal level correspondingto a falling transition, followed by a gradual decay to the zero signallevel, the transmitter being configured to combine the third signals anda signal representing the digital input signal to form a secondcomponent of a differential output pair.
 8. A transmitter comprising: afirst driver to generate a first signal representing a digital inputsignal; and a second driver having at least one high pass filter togenerate a second signal representing a high pass filtered version ofthe digital input signal, the high-pass filter including circuitelements having values selected to generate spikes in the second signalcorresponding to each transition of the digital input signal, each spikein the second signal having one of 1) a rapid rise from a zero signallevel corresponding to a rising transition, and 2) a rapid fall from azero signal level corresponding to a falling transition, followed by agradual decay to the zero signal level, the first and second signalsbeing combined to produce an output signal.
 9. The transmitter of claim8, wherein the first driver and the second driver are referenced to asame power supply voltage.
 10. The transmitter of claim 9, wherein ratioof an amplitude of the output signal to an amplitude of the first signalis independent of the power supply voltage.
 11. The transmitter of claim8, wherein the gradual decay follows at least approximately anexponential decay pattern.
 12. The transmitter of claim 8, wherein thesecond driver is a pulse-mode driver.
 13. The transmitter of claim 8,wherein the at least one high pass filter has at least one of anadjustable frequency cutoff and an adjustable gain.
 14. The transmitterof claim 8, wherein the at least one high pass filter has a frequencycutoff set to be about equal to a bit toggle frequency of the inputsignal.
 15. The transmitter of claim 8, wherein the at least one highpass filter includes a continuous-time high pass filter comprising aresistor-capacitor network.
 16. The transmitter of claim 8, wherein theat least one high pass filter includes an inverter, a resistor, and acapacitor connected in series.
 17. The transmitter of claim 8, whereinthe transmitter is configured to be coupled to a channel having afrequency attenuation, and wherein the second driver includes a set ofhigh-pass filters including the high-pass filter, the set of high-passfilters providing a filter transfer function to offset the frequencyattenuation.
 18. The transmitter of claim 8, wherein the first driverincludes a current mode differential driver configured to receive thesecond signal.
 19. A method comprising: generating a first signalrepresenting a digital input signal; generating a second signal, by atleast one high-pass filter, representing a high pass filtered version ofthe digital input signal, the second signal including spikescorresponding to each transition of the digital input signal, each spikein the second signal having one of 1) a rapid rise from a zero signallevel corresponding to a rising transition, and 2) a rapid fall from azero signal level corresponding to a falling transition, followed by agradual decay to the zero signal level, the at least one high-passfilter including circuit elements having values selected to generate thespikes; combining the first and second signals to produce an outputsignal; sending the output signal along a signal path to a receiver. 20.The method of claim 19, wherein the gradual decay follows at leastapproximately an exponential decay pattern.